The present invention relates to DNA sequences encoding insecticidal proteins, and expression of these sequences in plants.
Expression of the insecticidal protein (IP) genes derived from Bacillus thuringiensis (Bt) in plants has proven extremely difficult. Attempts have been made to express chimeric promoter/Bt IP gene combinations in plants. Typically, only low levels of protein have been obtained in transgenic plants. See, for example, Vaeck et al., Nature 328:33-37, 1987; Barton et al., Plant Physiol. 85:1103-1109, 1987; Fischoff et al., Bio/Technology 5:807-813, 1987.
One postulated explanation for the cause of low expression is that fortuitious transcription processing sites produce aberrant forms of Bt IP mRNA transcript. These aberrantly processed transcripts are non-functional in a plant, in terms of producing an insecticidal protein. Possible processing sites include polyadenylation sites, intron splicing sites, transcriptional termination signals and transport signals. Most genes do not contain sites that will deleteriously affect gene expression in that gene""s normal host organism. However, the fortuitous occurrence of such processing sites in a coding region might complicate the expression of that gene in transgenic hosts. For example, the coding region for the Bt insecticidal crystal protein gene derived from Bacillus thuringiensis strain kurstaki (GENBANK BTHKURHD, accession M15271, B. thuringiensis var. kurstaki, HD-1; Geiser et al. Gene 48:109-118 (1986)) as derived directly from Bacillus thuringiensis, might contain sites which prevent this gene from being properly processed in plants.
Further difficulties exist when attempting to express Bacillus thuringiensis protein in an organism such as a plant. It has been discovered that the codon usage of a native Bt IP gene is significantly different from that which is typical of a plant gene. In particular, the codon usage of a native Bt IP gene is very different from that of a maize gene. As a result, the mRNA from this gene may not be efficiently utilized. Codon usage might influence the expression of genes at the level of translation or transcription or mRNA processing. To optimize an insecticidal gene for expression in plants, attempts have been made to alter the gene to resemble, as much as possible, genes naturally contained within the host plant to be transformed.
Adang et al., EP 0359472 (1990), relates to a synthetic Bacillus thuringiensis tenebrionis (Btt) gene which is 85% homologous to the native Btt gene and which is designed to have an A+T content approximating that found in plants in general. Table 1 of Adang et al. show the codon sequence of a synthetic Btt gene which was made to resemble more closely the normal codon distribution of dicot genes. Adang et al. state that a synthetic gene coding for IP can be optimized for enhanced expression in monocot plants through similar methods, presenting the frequency of codon usage of highly expressed monocot proteins in Table 1. At page 9, Adang et al. state that the synthetic Btt gene is designed to have an A+T content of 55% (and, by implication, a G+C content of 45%). At page 20, Adang et al. disclose that the synthetic gene is designed by altering individual amino acid codons in the native Bt gene to reflect the overall distribution of codons preferred by dicot genes for each amino acid within the coding region of the gene. Adang et al. further state that only some of the native Btt gene codons will be replaced by the most preferred plant codon for each amino acid, such that the overall distribution of codons used in dicot proteins is preserved.
Fischhoff et al., EP 0 385 962 (1990), relates to plant genes encoding the crystal protein toxin of Bacillus thuringiensis. At table V, Fischhoff et al. disclose percent usages for codons for each amino acid. At page 8, Fischoff et al. suggest modifying a native Bt gene by removal of putative polyadenylation signals and ATTTA sequences. Fischoff et al. further suggest scanning the native Bt gene sequence for regions with greater than four consecutive adenine or thymine nucleotides to identify putative plant polyadenylation signals. Fischoff et al. state that the nucleotide sequence should be altered if more than one putative polyadenylation signal is identified within ten nucleotides of each other. At page 9, Fischoff et al. state that efforts should be made to select codons to preferably adjust the G+C content to about 50%.
Perlak et al., PNAS USA, 88:3324-3328 (1991), relates to modified coding sequences of the Bacillus thuringiensis cryIA(b) gene, similar to those shown in Fischoff et al. As shown in table 1 at page 3325, the partially modified cryIA(b) gene of Perlak et al. is approximately 96% homologous to the native cryIA(b) gene (1681 of 1743 nucleotides), with a G+C content of 41%, number of plant polyadenylation signal sequences (PPSS) reduced from 18 to 7 and number of ATTTA sequences reduced from 13 to 7. The fully modified cryIA(b) gene of Perlak et al. is disclosed to be fully synthetic (page 3325, column 1). This gene is approximately 79% homologous to the native cryIA(b) gene (1455 of 1845 nucleotides), with a G+C content of 49%, number of plant polyadenylation signal sequences (PPSS) reduced to 1 and all ATTTA sequences removed.
Barton et al., EP 0431 829 (1991), relates to the expression of insecticidal toxins in plants. At column 10, Barton et al. describe the construction of a synthetic AaIT insect toxin gene encoding a scorpion toxin using the most preferred codon for each amino acid according to the chart shown in FIG. 1 of the document.
The present invention is drawn to methods for enhancing expression of heterologous genes in plant cells. Generally, a gene or coding region of interest is constructed to provide a plant specific preferred codon sequence. In this manner, codon usage for a particular protein is altered to increase expression in a particular plant. Such plant optimized coding sequences can be operably linked to promoters capable of directing expression of the coding sequence in a plant cell.
Specifically, it is one of the objects of the present invention to provide synthetic insecticidal protein genes which have been optimized for expression in plants.
It is another object of the present invention to provide synthetic Bt insecticidal protein genes to maximize the expression of Bt proteins in a plant, preferably in a maize plant. It is one feature of the present invention that a synthetic Bt IP gene is constructed using the most preferred maize codons, except for alterations necessary to provide ligation sites for construction of the full synthetic gene.
According to the above objects, we have synthesized Bt insecticidal crystal protein genes in which the codon usage has been altered in order to increase expression in plants, particularly maize. However, rather than alter the codon usage to resemble a maize gene in terms of overall codon distribution, we have optimized the codon usage by using the codons which are most preferred in maize (maize preferred codons) in the synthesis of the synthetic gene. The optimized maize preferred codon usage is effective for expression of high levels of the Bt insecticidal protein. This might be the result of maximizing the amount of Bt insecticidal protein translated from a given population of messenger RNAs. The synthesis of a Bt IP gene using maize preferred codons also tends to eliminate fortuitous processing sites that might occur in the native coding sequence. The expression of this synthetic gene is significantly higher in maize cells than that of the native IP Bt gene.
Preferred synthetic, maize optimized DNA sequences of the present invention derive from the protein encoded by the cryIA(b) gene in Bacillus thuringiensis var. kurstaki, HD-1: Geiser et al., Gene, 48:109-118 (1986) or the cryIB gene (AKA Crya4 gene) described by Brizzard and Whiteley, Nuc. Acids. Res., 16:2723 (1988). The DNA sequence of the native kurstaki HD-1 cryIA(b) gene is shown as SEQ ID NO:1. These proteins are active against various lepidopteran insects, including Ostrinia nubilalis, the European Corn Borer.
While the present invention has been exemplified by the synthesis of maize optimized Bt protein genes, it is recognized that the method can be utilized to optimize expression of any protein in plants.
The instant optimized genes can be fused with a variety of promoters, including constitutive, inducible, temporally regulated, developmentally regulated, tissue-preferred and tissue-specific promoters to prepare recombinant DNA molecules, i.e., chimeric genes. The maize optimized gene (coding sequence) provides substantially higher levels of expression in a transformed plant, when compared with a non-maize optimized gene. Accordingly, plants resistant to Coleopteran or Lepidopteran pests, such as European corn borer and sugarcane borer, can be produced.
It is another object of the present invention to provide tissue-preferred and tissue-specific promoters which drive the expression of an operatively associated structural gene of interest in a specific part or parts of a plant to the substantial exclusion of other parts.
It is another object of the present invention to provide pith-preferred promoters. By xe2x80x9cpith-preferred,xe2x80x9d it is intended that the promoter is capable of directing the expression of an operatively associated structural gene in greater abundance in the pith of a plant than in the roots, outer sheath, and brace roots, and with substantially no expression in seed.
It is yet another object of this invention to provide pollen-specific promoters. By xe2x80x9cpollen-specific,xe2x80x9d it is intended that the promoter is capable of directing the expression of an operatively associated structural gene of interest substantially exclusively in the pollen of a plant, with negligible expression in any other plant part. By xe2x80x9cnegligible,xe2x80x9d it is meant functionally insignificant.
It is yet another object of the present invention to provide recombinant DNA molecules comprising a tissue-preferred promoter or tissue-specific promoter operably associated or linked to a structural gene of interest, particularly a structural gene encoding an insecticidal protein, and expression of the recombinant molecule in a plant.
It is a further object of the present invention to provide transgenic plants which express at least one structural gene of interest operatively in a tissue-preferred or tissue-specific expression pattern.
In one specific embodiment of the invention disclosed and claimed herein, the tissue-preferred or tissue-specific promoter is operably linked to a structural gene encoding an insecticidal protein, and a plant is stably transformed with at least one such recombinant molecule. The resultant plant will be resistant to particular insects which feed on those parts of the plant in which the gene(s) is(are) expressed. Preferred structural genes encode B.t. insecticidal proteins. More preferred are maize optimized B.t. IP genes.